The invention lies in the field of printing. The invention relates to a sheet-fed offset printing process, in which flow relationships in the interior of a housing of a sheet delivery of a sheet-fed offset printing machine are altered to prevent the uncontrolled spread of powder in the housing. Powder-laden air is extracted from the housing on at least one point along an empty run of a sheet conveying device and/or air is fed into the housing from outside. The invention relates further to a sheet-fed offset printing machine having a sheet delivery and a powdering device disposed in a housing of the sheet delivery to powder the printed sheets. The machine has air feed devices to feed air into the housing from outside and/or air extraction devices to extract powder-laden air from the housing. The devices are disposed on at least one point along an empty run of a sheet conveying device to prevent the uncontrolled spread of powder in the housing by altering the flow relationships inside the housing.
Printing processes and printing machines are described, for example, in my German patent application DE 198 01 949.1, from which priority is claimed in my U.S. patent application Ser. No. 09/619,975 through PCT/EP99/00271. U.S. patent application Ser. No. 09/619,975 is hereby incorporated herein by reference. The application proposes to feed air deliberately into the interior of the housing of a sheet delivery of a sheet-fed offset printing machine to prevent the uncontrolled spread of powder in the interior of the delivery or into the surroundings by altering the flow relationships. With a view to limiting the spread of powder and reducing the necessary amounts of powder, particularly good results are achieved if the air was fed into the housing from above through the empty run in a region located downstream of a powdering device in the direction of motion of a conveying run of the sheet delivery, and/or at the same time powder-laden air was extracted from such region.
In the sheet deliveries of conventional offset printing machines, air turbulence occurs to a great extent. Various effects cause the air turbulence. For example, underneath and above the conveying run and the empty run of the sheet delivery, the suction action of the moving sheet grippers and chains entrains air in the direction of motion of the respective run. Also, printed sheets carried along by the conveying run flutter or, when being deflected from an oblique angle into the horizontal, flip up at their rear edge (also known as the lashing or flag effect). Further, air is blown from below against the sheet from the sheet guide plate to guide the sheet over the guide plate without contact, but some of the air flows upward between the adjacent sheets and laterally past the sheets.
As a result of the air turbulence, a large part of the powder emerging from the powdering device is swirled and entrained by the air. Thus, it does not pass as desired onto the printed surface of the sheets directly underneath the powdering device. Instead, the powder is distributed over the entire interior of the sheet delivery by the air flows, in particular, those along the two runs of the sheet delivery. The distribution not only leads to increased wear of moving parts and relatively long down times for cleaning the sheet delivery, it also aggravates operating personnel because the swirled powder emerges into the surroundings through all the openings in the sheet delivery, in particular, through the sheet outlet opening and air outlet grating on the top of the housing.
In addition, as a result of the powder being transported back as far as the impression cylinder of the last printing unit or as far as the delivery drum of the printing machine, powder gets onto the still moist surface of the printing material or the printing ink directly after the printing material has emerged from the printing machine. The powder, which usually includes starch, picks up part of the liquid present there, for example, surface water originating from the feed of damping solution, and possibly also relatively highly volatile or low-viscosity constituents of the printing ink. Because the liquid picked up by the powder is not absorbed into the printing material, or is absorbed considerably more slowly, and does not participate in the chemical crosslinking process of the printing ink either, the moisture content of the sheets is higher when they are deposited on the sheet stack. Such moisture delays the drying, and there is an increase in the time intervals needed before the further processing of the printed products. Next, in the event shear is exerted on the moist printing ink, the liquid picked up by the powder can also have a decreasing effect on the shear strength of the printing ink if deposited powder particles are detached. Such detachment can result in the exposure of incompletely dried point-like surface areas of the printing ink located underneath the detached particles.
The adverse affect is counteracted, according to my co-pending U.S. patent application Ser. No. 09/619,975, by feeding air into the housing of the sheet delivery from above through the empty run. The air feed firstly prevents powder from being transported in the direction of the printing machine by the empty run and by air flows along the empty run and, secondly, in conjunction with one or more brush strips, the empty run being freed from adhering powder. If some of the air fed in, after passing through the empty run, is deflected in the direction of the printing machine, the deflected air also prevents powder in the region of the powdering device from getting as far as the empty run of the sheet delivery and from being transported by the empty run in the direction of the printing machine. As a result, at least in the case of sheet-fed offset printing machines with short sheet deliveries, the region of the sheet delivery located directly downstream of the delivery drum could be kept virtually powder-free, a status that not only reduces wear and powder emissions into the surroundings but, surprisingly, could also accelerate the drying of the printing material and the printing ink, probably because of the action mechanism described in the preceding paragraph.
The amount of excess powder emerging into the surroundings of the sheet delivery could be reduced once more by extracting part of the air fed in and of further powder-laden air from the interior of the housing of the sheet delivery in the direction of movement of the conveying run downstream of the powdering device, as proposed by my co-pending U.S. patent application Ser. No. 09/619,975.
Trials and experience with processes and devices for keeping clean or cleaning a sheet delivery of a sheet-fed offset printing machine of excess powder according to my U.S. patent application Ser. No. 09/619,975 have shown, however, that further improvements are possible with regard to reducing the powder emissions inside and outside the sheet delivery.
It is accordingly an object of the invention to provide a sheet-fed offset printing process and a sheet-fed offset printing machine that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that reduces the spread of powder.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a sheet-fed offset printing process, including the steps of altering flow relationships in an interior of a housing of a sheet delivery of a sheet-fed printing machine to prevent uncontrolled spread of powder in the housing by at least one of extracting powder-laden air from the housing on at least one point along an empty run of a sheet conveying device; and feeding air into the housing from outside the housing on at least one point along the empty run, channeling air flows inside the housing by feeding air not laden with powder into the housing and leading the air not laden with powder into an air flow path along the empty run downstream of the at least one point with respect to a direction of motion of the empty run to transport the air not laden with powder in a direction toward a printing unit of the printing machine by at least one of a suction effect of the empty run, and at least impeding an inflow of powder-laden air from a region of at least one of a conveying run of the sheet conveying device and a powdering device into the flow path along the empty run.
The channeling or control of air flows within the housing according to the invention makes it possible to prevent the air flows (which to some extent are only produced by altering the flow relationships in the housing as a result of feeding and/or extracting air along the empty run of the sheet conveying device) from transporting swirled powder as far as the impression cylinder of the last printing unit or as far as the delivery drum of the printing machine.
The invention is based on the finding that the principle transport paths for swirled powder in the housing of the sheet delivery run along the sheet conveying device because of the suction action of the sheet conveying device, the air flow along the empty run transporting powder in the direction of the printing machine. As a result of feeding and/or extracting air along the empty run, as described in my co-pending U.S. patent application Ser. No. 09/619,975, a large part of the entrained powder can be eliminated, in particular, if, according to a preferred configuration of the invention, the feeding and/or extraction of air produces an air flow that passes through the empty run transversely with respect to its direction of movement. Such a configuration has the effect of interrupting the powder-laden air flow along the empty run. However, a vacuum is produced downstream of the interruption point, which can lead to powder-laden air being extracted from other regions of the sheet delivery. In addition, between the opposed air flows along the empty run and the conveying run of the sheet conveying device, transverse flows are formed, which can likewise result in transporting swirled powder from the region of the powdering device and the conveying run into the flow path of the empty run and, from there, to the impression cylinder of the last printing unit or to the delivery drum of the printing machine.
In order to prevent such a result, the channeling of the air flows inside the housing according to the invention, firstly in a direction of motion of the empty run downstream of the point at which the air is extracted and/or fed in, air not laden with powder is fed into the housing to carry away powder-laden air entrained along the empty run by the suction action of the empty run from the flow path of the empty run.
In accordance with another mode of the invention, the air not laden with powder is fed in from above the empty run into the air flow path along the empty run.
The preferably pre-cleaned air replaces the powder-laden air carried away and prevents powder-laden air subsequently flowing from other regions of the sheet delivery because of a vacuum, which can occur if the flow path along the empty run is interrupted. Second, with the channeling of the air flows according to the invention, the inflow of powder-laden air from the region of a powdering device or a conveying run of the sheet conveying device into the flow path along the empty run can additionally or alternatively be impeded or blocked by mechanical or fluidic devices to reduce the entry of powder from other regions of the sheet delivery or to prevent it completely.
The air not laden with powder can be fed in by air either being blown into the housing from outside and/or by being drawn in from outside by a vacuum generated in the housing as a result of interrupting the powder-laden air flow along the empty run. The configuration has a first advantage that the amount of air blown to interrupt the air flow and/or for flushing purposes can be kept small, and a second advantage that powder particles cleaned off and possibly contained in the air are prevented from being transported in the direction of the printing machine.
In accordance with a further mode of the invention, the amount of air fed into the housing is controlled by at least one of a printing speed of the printing machine and a running speed of the sheet conveying device.
In accordance with an added mode of the invention, at least some of the air is fed into the housing at atmospheric pressure and the air is drawn into the housing with a vacuum generated in the housing. Preferably, the air from an air reservoir is disposed on a top side of the housing.
The extraction of air is carried out in the simplest case through air slots or ventilation grilles in the top of the housing, through which air emerges from the interior of the sheet delivery in conventional printing machines. The air blown in and/or extracted is deflected in the direction of the printing machine as it enters the housing and then (as does the powder-laden air in conventional sheet deliveries) flows along the empty run of the sheet conveying device, as a result of the suction of the sheet conveying device, as far as the delivery drum of the printing machine. Because the housing of the sheet delivery is closed in the region, the air is deflected there together with the sheet conveying device and flows in the direction of the powdering apparatus again, underneath and above the sheets transported by its conveying run. The air flow forms an air cushion above and below the freshly printed sheets which, if necessary, can be somewhat amplified on the underside by feeding air through the sheet guide plate, and ensures an extremely quiet run of the sheets.
The virtually complete absence of powder in the air cushion, that is to say, in the vicinity of a portion of the delivery run of the sheet conveying device located between the printing machine and the powdering apparatus, has the effect of permitting powder only to get to the surface of the moist printing material and the moist printing ink when the latter has already been pre-dried somewhat as a result of the absorption of water and liquid binder constituents of the printing ink. Thus, less liquid can be picked up or bound by the powder. In addition, the powder does not remain stuck to a pre-dried surface of the printing ink, which, in conventional printing processes without additional UV or IR drying, would often lead to losses in terms of quality, in particular, on highly glossy prints.
Quieting the sheet run and reducing the amount of air that has to be blown in from below through the sheet guide plate in turn results in a reduction of the air turbulence in the region of the powdering apparatus, so that the powder is swirled less there and, therefore, gets to the surface of the sheet.
In accordance with an additional feature of the invention, the inflow of powder-laden air from the region of at least one of the conveying run and the powdering device into a flow path of the empty run is blocked by deflecting at least some of the air fed in through the air flow path along the empty run in a downward direction past a side of the powdering device facing the printing machine towards sheets transported by the conveying run past the powdering device.
In accordance with yet another mode of the invention, cleaned surrounding air is fed into the housing of the sheet delivery.
In accordance with yet a further mode of the invention, the air flow path running along the empty run of the sheet conveying device in the direction toward the printing unit is interrupted by at least one of extracting the powder-laden air and feeding air into the housing from outside on at least one point between a free end of the sheet delivery and the printing unit. Preferably, the air flow path is interrupted by at least one of at least one row of brushes, and air flowing transversely with respect to the direction of motion of the empty run.
In accordance with yet an added mode of the invention, at least one of the powder-laden air and the air not laden with powder fed into the housing from outside is extracted in a horizontal portion at a top side of the sheet delivery.
In accordance with yet an additional feature of the invention, the inflow of powder-laden air is blocked from the region of at least one of the conveying run of the sheet conveying device and the powdering device into a movement path of the empty run with a dividing wall disposed between the empty run and the conveying run.
A mechanical device or means for blocking an inflow of powder-laden air include, in particular, a dividing wall that is disposed between the empty run and the conveying run and which expediently extends over the entire width of the sheet delivery and at least over part of the length of the sheet delivery, preferably beginning at the place where the air is extracted and/or fed in along the empty run or the powder-laden air flow is interrupted. The wall then runs into the vicinity of the delivery drum of the printing machine.
Fluidic devices or means for impeding an inflow of powder-laden air into the flow path along the empty run include, in particular, extracting powder-laden air, feeding in air not laden with powder or generating targeted air flows which prevent the entry of powder-laden air into the flow path of the empty run.
With the objects of the invention in view, there is also provided a sheet-fed offset printing machine for printing sheets, including at least one printing unit, a sheet conveying device having an empty run and a conveying run, the empty run having a motion direction, the sheet conveying device connected to the at least one printing unit, a sheet delivery having a housing defining an interior, the sheet delivery connected to the sheet conveying device, a powdering device disposed in the housing for powdering printed sheets, at least one of air feed devices feeding air into the housing from outside the housing and disposed on at least one point along the empty run to prevent an uncontrolled spread of powder in the housing by altering flow relationships inside the housing, and air extraction devices for extracting powder-laden air from the housing and disposed on at least one point along the empty run to prevent an uncontrolled spread of powder in the housing by altering flow relationships inside the housing, and channeling devices at least one of leading air not laden with powder, fed into the interior of the housing downstream of the at least one of the air extraction devices and the air feed devices in the motion direction, into an air flow path along the empty run to transport the air not laden with powder in a direction of the at least one printing unit by suction, and at least impeding an inflow of powder-laden air from a region of at least one of the conveying run and the powdering device into the air flow path along the empty run.
In accordance with again another feature of the invention, the channeling devices include at least one air opening in the housing and/or at least one air reservoir communicating with the interior of the housing.
In accordance with again a further feature of the invention, the air opening and/or air reservoir opens into the housing in a vicinity of the empty run and/or communicates with the interior of the housing.
In accordance with again an added feature of the invention, the air opening and/or air reservoir is disposed on the top side of the housing above the empty run.
In accordance with again an additional feature of the invention, there is provided a delivery drum connected to the sheet conveying device. The air opening and/or air reservoir is disposed between the delivery drum and the air extraction devices and/or the air feed devices.
In accordance with still another feature of the invention, powder-laden air is extracted in the vicinity of the powdering apparatus, preferably at the rear edge (facing the printing machine) of an air extraction and deflection trough used for air extraction and air feed. The configuration prevents the undesired swirling of powder away from the powdering apparatus as a result of air turbulence in the vicinity of the latter and, at the same time, deflects part of the fresh air fed into the housing of the delivery downward in the direction of the printed sheets after passing through the empty run in the region of the powdering apparatus. Above the powdering apparatus, an air diverter can expediently be disposed, which deflects some of the air fed in downward past the front side of the powdering apparatus, on the delivery side, and the rear side of the powdering apparatus, on the printing-machine side. The partial flow deflected toward the front side is preferably part of the fresh air used to interrupt the air flow and blown in from above through the empty run, while the partial flow deflected toward the rear side is part of the air drawn into the housing of the sheet delivery from outside downstream of the interruption point.
In accordance with still a further feature of the invention, the powdering device has a side facing the at least one printing unit and another side facing away from the at least one printing unit, and an air diverter is disposed under the at least one air opening or air reservoir dividing air fed into the housing from outside the housing into two partial-flows, a first of the two partial-flows directed along the side of the powdering device facing the at least one printing unit and a second of the two partial-flows directed along the another side facing away from the at least one printing unit in a direction of sheets passing underneath the powdering device.
In accordance with still an added feature of the invention, an amount of air fed into the housing is to be controlled by altering one of a printing speed of the printing machine and a running speed of the sheet conveying device.
In accordance with still an additional feature of the invention, there are also provided air cleaning devices for cleaning air fed into the housing.
In accordance with another feature of the invention, the channeling devices include a dividing wall disposed over a part of the length of the sheet delivery between the empty run and the conveying run.
In accordance with a further feature of the invention, the dividing wall extends into a vicinity of the at least one printing unit.
In accordance with an added feature of the invention, the dividing wall extends from the air feed devices and/or the air extraction devices into a vicinity of the at least one printing unit.
In accordance with an additional feature of the invention, the sheet delivery has an upper horizontal portion; and some of the air feed devices and/or the air extraction devices are disposed in the upper horizontal portion.
In accordance with yet another feature of the invention, the air feed devices and/or the air extraction devices produce an air flow passing through the empty run transversely with respect to the motion direction of the empty run to interrupt a powder-laden air flow running along the empty run in a direction towards the at least one printing unit.
In accordance with yet a further feature of the invention, there is provided at least one brush configuration at least partly blocking the air flow path along the empty run.
As a result of the above measures for preventing the spread of powder inside the housing of the sheet delivery, it is additionally possible for the powder consumption to be reduced to an extreme extent, that is to say, down to about 10% of the amounts previously needed. The reduction first results because the powder substantially only arrives where it is needed, namely on the surface of the sheets, and also results due to the fact that there is considerably less powder needed to prevent set-off the sheets because of the previously effected superficial drying of the printing ink and of the printing material. Lower powder consumption at the same time also means lower powder emissions into the surroundings of the sheet delivery, so that the powder emission problem can be largely eliminated.
In the region of the air outlet opening, further air extraction devices can additionally be provided. These devices can be used to extract powder between the edges of the sheet outlet opening and the edges of the printed sheets before the powder is blown downward through the sheet outlet opening into the surroundings of the sheet delivery by the delivery fans. Such an addition achieves a further reduction in the powder emissions into the surroundings.
In long sheet deliveries, in particular, in deliveries possibly fitted with IR dryers or other drying devices, it may be preferable to interrupt the air flow several times by air extractions, air feeds, and/or brush configurations and in each case then to feed air not laden with powder into the flow path of the empty run to ensure that the spread of powder is prevented and the clean air fed in gets as far as the printing machine. For such a purpose, it is expediently possible for substantially the same devices to be used as the first devices that are used to interrupt the air flow and to feed in air not laden with powder. The devices are expediently disposed in the upper horizontal portion of the sheet delivery. The devices preferably include a nozzle box, possibly fitted with brushes, disposed above the empty run, from which clean air is blown downward in the direction of the empty run, an air extraction and deflection trough that is disposed underneath the nozzle box in the interspace between the empty run and the conveying run and possibly fitted with brushes and that, in each case, extracts some of the air or deflects it in the direction of the printing machine, and an air reservoir that is disposed downstream of the nozzle box and the trough in the running direction of the empty run, and from which the vacuum generated draws clean air into the housing of the sheet delivery through air openings.
Depending on the length and configuration of the sheet delivery, it may be sufficient, instead of one of the aforementioned devices, that is to say, nozzle box, trough, and air reservoir, to provide only an air reservoir on the top of the housing. Thus, if required, further air can be drawn into the housing of the sheet delivery from outside by the suction of the empty run.
In accordance with a concomitant feature of the invention, air fed in through the air opening and/or air reservoir is at atmospheric pressure.
The air reservoir can be kept under a slight positive pressure, so that the air is always fed into the interior of the housing of the sheet delivery. However, the air in the interior of the air reservoir is preferably under atmospheric pressure. According to a further preferred configuration or alternative of the invention, simple control of the feed of air as a function of the printing speed is made possible because higher printing speeds necessitate higher running speeds of the sheet delivery, which, in turn, causes a more intense suction action, as a result of which more air is drawn out of the air reservoir.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a sheet-fed offset printing process and a sheet-fed offset printing machine, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.